The present invention relates to paper and board making machines in general, and more particularly to coating stations in such machines. The term coating station is meant to include a sizer or sizing station in which size is applied to the paper or board surface or surfaces in the drying section of a paper or board machine.
In coating paper or board web, either direct or indirect application can be used. In direct application, coating material is spread on the web as it moves past an application area on the surface of a counter roll. In indirect application, coating material is first spread on a film-transfer roll or other transfer surface from which it is transferred onto the web in a coating nip. In direct application it is further possible that the web passes through a size or coating bath which is formed between two rolls and then runs through a nip which is formed between the two rolls and which forms the lower outlet of the size or coating bath. Coating material is often applied excessively and the amount of coating that remains on the moving substrate is adjusted, i.e., coating is metered by means of a doctoring element. A doctor blade or a rotating doctor rod can be used as the doctoring element which is pressed against the moving substrate. The load of the doctoring element is provided by means of loading hoses operating with a pressure medium or a mechanical force driving the doctoring element. It depends on the load of the doctoring element how much coating material is carried away from the application area along with the moving web. There are also contact-free coating methods that require no doctoring element. In these methods, all of the applied coating material remains on the web, whereby the metering of coating takes place by means of a pump or a valve that feeds coating material into the application device.
However, accurate measurement and control of the coating quality has become increasingly difficult with the increase of machine width and speed which are nowadays used to improve the paper machine performance. In particular, typical CD-scanning devices employing optical measurement methods in scanning measuring heads suffer from an immanent inaccuracy of the measured values with respect to the web movement; i.e. the measured values are far away from a snap-shot of a CD-profile of measured coating quality.
FI 20055019 (WO 2006/075055) discloses a method for measuring a nip pressure and/or a pressure profile, wherein a nip-forming roll is fitted with a pressure sensor under a cover layer which is protective and functions as a spring on top of the sensor, and the nip pressure delivers to the bottom surface of the cover layer a deformation proportional to the nip pressure and the deformation compresses the sensor, which responds to the deformation by generating an electric signal.
FI20055020 (WO 2006/075056) discloses a method for measuring a nip pressure and/or a pressure profile, wherein a nip-forming roll is fitted with a pressure sensor under a cover layer which is protective and functions as a spring on top of the sensor, and the nip pressure delivers to the bottom surface of the cover layer a deformation proportional to the nip pressure and the deformation compresses the sensor, which responds to the deformation by generating an electric signal.
Metso Paper, Inc. markets an intelligent roll, known as the iRoll™ tension measuring roll which automatically measures tension profiles and their quality online. FI20055019 (WO 2006/075055) (iRoll tension profile measuring) and FI20055020 (WO 2006/075056) (iRoll pressure profile measuring) generally focus on measuring CD profiles.
Pressure-sensitive electromechanical film (EMFi) film has been used as a microphone and a speaker. There are applications, inter alia, in active noise reduction. See e.g. http://www.panphonics.fi/applications.html. The speaker/noise reduction applications are based on the vibrating ability of the film. As a measuring sensor or a microphone, the film converts pressure/sound into an electric signal, but as a speaker it converts the electric signal to vibration. The technique is cost-effective so that it may be utilized in covering large wall/ceiling areas e.g. for adjusting noise in offices, between common and private premises.
It is known to measure the position of the edge of the web or the wire by a pressure-sensitive film sensor. It is known to measure the tension and linear-load profile by a film sensor. It is known to place a pressure-sensitive film sensor behind a metal plate in the key pad of an elevator to identify the touch of a finger on the surface of the metal plate.
What is needed is a method and an apparatus for quickly and accurately determining quality characteristics or details of a coating applied in paper or board coating stations. This method and apparatus shall be applicable to coating stations of types in which a coating nip is present or absent.